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OT - Converting Electric Motor

S

skycarp

Member
Hi Guys,

What does it take to convert a used 5 HP 3 phase electric motor to a 2 phase motor, to be used on a large commercial air compressor.

I have access to several used motors but they are all three phase, and I do not have 3 phase power to my shop.

Thanks for any help on this problem.
 
You can't convert a 3 phase motor to single phase, but if you have another 3 phase motor thats 7.5 hp or larger you could build yourself a rotary phase converter for next to nothing.

Plenty of info on the net about how to do it so I wont explain it here. Google it.

Jeff
 
Yup - single phase & 3-phase are just built different, if you want single phase you need to get a single phase motor. They need bigger wires to they cost more.

As mentioned, you can use a spare 3-phase motor to make a phase converter so you can input single phase and it will output 3-phase to run your 3-phase motor.

There are also electronic controlers that will do the phase conversion.

So you can either replace the 5 hp motor, or use a phase converter to change the single phase into 3-phase.

(There isn't 2-phase, conventional 120 or 220v are called single phase....)

--->Paul
 
I think a 3 phase converter will give you about 70 % of the motor's hp. It has to do with power factors and efficiency. My dad used to make converters for farmers. He would take a lathe and shave off some of the armature on the motor used to make the 3 phase. He also make static converters using 2 different banks of capacitors. One bank was the starting capacitors and the other were the running capacitors. I forget what the rule was for capacitor size/hp. He would use adjustable timers and motor contactors to connect the start caps for about 1 second and then another motor contactor to pick up the run caps. After you got the motors under load, it was a matter of fine tunning the total capacitor size to minimize the current draw. He would also install bleeder resistors to drain the caps before restarting the motor.
 
google "homemade phase converters". You do not need capacitors, you need a three phase motor at least as big as the one you want to run, plus a small- maybe 1 hp single phase to start it spinning...(then shut it off)... wire it according to instructions, and the three phase, hooked to 240 volt single phase, will generate the third leg to give you three phase power. Made mine years ago, runs an 8 foot bed 18 inch swing turning lathe, milling machine, and hacksaw....all 3 phase. Cost was zilch...bought one switchbox, rest was around the shop. Rotary converter is basically the same thing as what I built, except the RC has no outside shafts, like the 3 phase motor, but has capacitors, to make it spin...but the 1 hp single phase motor I use does the same thing.
 
I would not run a 3 phase air compressor on a phase converter. It would run fine I would just not want the phase converter up and running all the time when your air compressor was cutting off and on. you are just spending money for energy usage you don't need to if your compressor gets up to max pressure and shuts off and on as you use it. get the right single phase motor.
 

I am interested in your offer for sale of a used 5 hp single phase motor. I do not know what you value this motor at so I can not make you an offer at this time.

Please set a price you are comfortable with and I will consider it. Also, please let me know where you are located so I can consider shipping costs.

thanks,

Dean
deand1 at kanab dot net
 
By the time all is said and done with all this phase converter stuff, it'll be cheaper and quicker to get a NEW 5HP compressor motor.

To make the "cheap" phase converter work you need a second 3-phase motor, and ANOTHER regular motor to spin the second motor so it will "generate" 3-phase electricity.

Unless you are getting the 3-phase motor CHEAP, and can source all the other parts for FREE, there's not much point in it.

You need a compressor motor, right? Remember they run 3450RPM.

Northern Tool has NEW 5HP compressor motors for $289.
 
Try running a 5HP motor at full rated amps continuous from a 5 or even a 7-1/2 HP rotary converter motor.
Keep a C02 fire extinguisher handy.
 
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